As known, charcoal is the solid carbon residue resulting from pyrolysis of carbonaceous materials having a cellulosic fiber structure. These include woody materials and vegetable biomass, such as tree stock, tree bark, twigs, sawdust, and even nut shells, as well as processed materials, such as corrugated paper board and paper. Depending on whether the produced charcoal is a commercial product or the byproduct of solid waste disposal, its intended use determines the type of carbonaceous material used as the source charge.
The common industrial use of charcoal is as a fuel. In many instances it is used as a fuel for outdoor cooking, but in countries with limited fossil fuel reserves it is commonly used as a fuel source in commercial manufacturing applications. One such application is in the production of pig iron, an early product of the overall steel manufacturing process. There it is used to fuel blast furnaces in integrated mills which produce the iron from iron ore. These type of industrial applications have a high energy demand and consume large quantities of fuel. Since charcoal does not have the heat content of coke its use in these applications consumes a proportionately higher volume of charcoal. This higher volume consumption places a high demand for lower unit cost charcoal, which in turn places high priority on both the efficiency and scale of industrial charcoal production methods and equipment.
The source materials for industrial charcoal are medium to dense raw hardwoods, such as beech, birch, hard maple, hickory, and oak. Others are softwoods; primarily long leaf and slash pine. The current charcoal manufacturing kilns are either batch process or continuous multiple hearth kilns. Due to the higher labor requirements of the batch kilns, which are manually-loaded and -unloaded, continuous hearth kilns are more commonly used. In addition to their limited labor, the continuous kilns also provide a higher produced charcoal volume than batch kilns. Existing batch kiln, such as the Missouri-type charcoal kilns producing charcoal briquettes, are small kilns which typically produce less than 20 tons of charcoal over a 3-week cycle. In contrast, continuous hearth units produce an average of 2 to 3 tons of charcoal per hour.
In blast furnace applications the charcoal to iron ratio is approximately 0.6 tons charcoal to one ton iron. Therefore, an annual mill production of 375, 000 tons of iron requires approximately 225, 000 tons of charcoal. Satisfying this need with the prior art batch kilns having a best case annual yield of 350 tons, would require more than 640 kilns. Similarly, satisfying the demand with continuous hearth units, each producing 16,500 tons of charcoal annually at a 75% duty cycle, requires 14 kilns.
Apart from considerations on the per unit time volume of charcoal produced, there is also concern with regard to disposal of the toxic gas discharges produced by pyrolysis. During the manufacturing process the wood is heated, driving off water and highly volatile organic compounds (VOC). The wood temperature rises to approximately 275° C. (527° F.) and the VOC distillate increases. At this point, external heat is no longer required because the carbonization reactions become exothermic. At 350° C. (662° F.), exothermic pyrolysis ends, and heat is again applied to remove the less volatile tarry material byproducts from the product charcoal.
The byproducts include noncondensible gases (carbon monoxide [CO], carbon dioxide [CO2}, methane, and ethane), pyroacids (primarily acetic acid and methanol), tars and heavy oils, and water. All of these are emitted with the kiln exhaust and emission controls combust the organic compounds (ethane, methane, ethanol, and polycyclic organic mailer (POM)) and CO to CO2 and water before emitting it to the atmosphere.
Continuous production of charcoal is more amenable to emission control than batch production because emission composition and flow rate are relatively constant, and emissions may be controlled with afterburners. Control of emissions from batch kilns is difficult because the process and emissions are cyclic, with the emission composition and flow rate changing over the process cycle. Batch kilns do not typically have emission control devices.
There is therefore, a present need for a process and apparatus capable of producing higher volumes of charcoal per unit time, and at lower unit cost, while also providing effective emission control of the toxic constituents of the production process.